Welding by radiofrequency (RF) energy is an efficient and fast way to manufacture certain products. For example, radiofrequency energy may be used to weld polymeric material, such as polyvinyl chloride (PVC), to make flexible bags for retaining fluid. For example, a bag for receiving pressurized air is incorporated into a vascular compression device for preventing pulmonary embolisms and deep vein thrombosis (DVT).
A bag of a typical vascular compression device includes a pair of opposing polymeric sheets, such as PVC, welded around their perimeters and a polymeric tube port, such as PVC, welded between the sheets in fluid communication with the bag. An exemplary conventional process for forming the bag uses a die for welding the bags together and a cylindrical mandrel for welding the polymeric sheets to the tube. The cylindrical mandrel is inserted into the tube, and the mandrel, the tube and the opposing sheets are compressed between the dies. Radiofrequency energy is supplied to the die to create a radiofrequency electric field. The electric field heats the polymeric sheets, thereby welding the sheets together form the perimeter of the bag.
After the bag is welded, the radiofrequency energy directed to the dies is stopped. Radiofrequency energy is then supplied to the mandrel so that the radiofrequency electric field is directed outward in a radial direction from the mandrel, through the tube and sheets. The radiofrequency electric field heats the tube and the sheets, thereby welding the sheets to the tube and welding the tube in fluid communication with the bag.
The use of a cylindrical mandrel may be inefficient and time-consuming because of the difficulties in both inserting the mandrel into the tube and removing the mandrel from the tube after the process.
In another exemplary process, the mandrel is replaced by a rigid, non-deformable tubular insert that is received in the tube. Like the above process, radiofrequency energy is supplied to a die, for example, to create an electric field. However, in this process, the die includes portions that surround the tube and the tubular insert and direct the radiofrequency electric field into the tube and the sheets surrounding the tube to weld them together.
Although this process purportedly welds both the bag and the tube to the bag at the same time and in one step, the use of a tubular insert, without more, is not sufficient to weld both the bag and the sheets to the tube. Welding the sheets to the tube takes longer than welding the bag because the tube is typically thicker than the polymeric sheet. If the process lasted long enough to adequately weld the sheets to the tube, then there is a risk that the die will cut or at least weaken the bag at the bag perimeter because of the amount of time the sheets would be subjected to the electric field.